1. Field of the Invention
This invention relates to a transistor epitaxial wafer and, in particular, to a structure of an n-type InGaAs non-alloy layer thereof. Also, this invention relates to a transistor produced by using the transistor epitaxial wafer.
2. Description of the Related Art
A high-frequency device using a compound semiconductor such as GaAs is widely used for an amplifier etc. of a mobile phone and other communication devices since it can provide frequency characteristics with low distortion and good efficiency at GHz or more. Above all, a heterojunction bipolar transistor (hereinafter referred to as “HBT”) using a heterojunction for an emitter/base junction has excellent frequency characteristics and is widely used as a high-output transistor for a mobile phone since its emitter layer has wider bandgap than its base layer to allow a high emitter injection efficiency.
The emitter/base junction of the HBT has been generally composed of an AlGaAs/GaAs heterojunction. However, in recent years, the AlGaAs emitter layer is positively replaced by an InGaP emitter layer in order to enhance the device characteristics or reliability.
FIG. 7 illustrates the structure of a conventional HBT. The HBT 300 is an InGaP/GaAs-based HBT. An epitaxial wafer used in the HBT comprises, grown on a semi-insulating GaAs substrate 1 by a vapor-phase epitaxy such as MOVPE and MBE, an n-type sub-collector layer 2, an n-type collector layer 3, a p-type base layer 4, an n-type InGaP emitter layer (or an n-type AlGaAs) 5, an n-type GaAs emitter contact layer 6, and an n-type InGaAs non-alloy layer 14.
Silicon (Si) is generally used as an n-type dopant, and carbon (C), zinc (Zn) or beryllium (Be) is used as a p-type dopant element.
In the HBT 300, an emitter electrode 11 is formed on the n-type InGaAs non-alloy layer 14, a base electrode 12 is formed on the p-type base layer 4, and a collector electrode 13 is formed on the n-type sub-collector layer 2. In operation with the emitter grounded, positive voltages Vb, Vc are applied to the base electrode 12 and the collector electrode 13, respectively, and base current Ib as a signal input is fed from the base electrode 12 to control collector current Ic as an output to operate as a transistor.
In order to reduce the power consumption of the HBT 300, it might be assumed to lower a resistance component of the following portions (1) to (5).    (1) Contact resistance of the n-type InGaAs non-alloy layer 14 contacting the emitter electrode 11.    (2) Contact resistance of the base layer 4 contacting the base electrode 12.    (3) Contact resistance of the sub-collector layer 2 contacting the collector electrode 13.    (4) Resistance in each the epitaxial layers.    (5) Interface resistance between the epitaxial layers.
Above all, the contact resistance of the n-type InGaAs non-alloy layer 14 contacting the emitter electrode 11 as mentioned in (1) is likely to increase since the size of the n-type InGaAs non-alloy layer 14 is smaller than the electrode size of the base layer 4 and the sub-collector layer 2. In consideration of this, the n-type InGaAs non-alloy layer 14 has an In composition as high as 0.4 to 0.7 and it is doped at a high doping concentration of 1×1019 cm−3 or more.
On the other hand, when the n-type InGaAs non-alloy layer 14 with a high In composition is directly grown on the GaAs emitter contact layer 6, a number of defects are generated at the interface of the GaAs and the InGaAs since the lattice constant of InGaAs of the non-alloy layer 14 is significantly different from that of the GaAs layer 6. Thus, in order to reduce the number of the defects as much as possible, the n-type InGaAs non-alloy layer 14 is formed of a linear graded layer (i.e., an In0→xGa1→1-xAs linear graded layer) where the In composition is gradually increased from 0 to a desired uniform In composition value (a final value), and then a uniform In composition layer (i.e., an InxGa1-xAs non-alloy layer with a uniform In composition) is grown thereon which has an In composition fixed to the final value.
Further, when Si is used as an n-type dopant to obtain the n-type InGaAs non-alloy layer 14 with a low resistance, it is difficult to obtain the layer 14 with a carrier concentration of 1×1019 cm−3 or more since the doping efficiency of Si is lower than that of Se or Te. Thus, although it is needed to use the more dopant to have the high carrier concentration, the growth temperature of the n-type InGaAs non-alloy layer 14 must be increased to do that. Because of this, the surface flatness of the n-type InGaAs non-alloy layer 14 will deteriorate. On the other hand, when Se or Te with high doping efficiency is used as the n-type dopant, the surface flatness of the n-type InGaAs non-alloy layer 14 can be improved and the contact resistance thereof can be reduced. However, the Se or Te may diffuse into the emitter layer 5 during the growth since the Se or Te has a higher diffusion coefficient than Si.
JP-A-2003-133325 teaches a non-alloy layer structure constructed such that, as shown in FIG. 7, the n-type InGaAs non-alloy layer 14 comprises a linear graded layer 15 having an In composition varied linearly from 0 to 0.5 and a non-alloy layer (i.e., a uniform composition layer) 16 formed thereon with a fixed In composition of 0.5, and Se is doped into the upper uniform composition layer 16. Thus, the surface flatness of the n-type InGaAs non-alloy layer 14 can be improved and the contact resistance thereof can be reduced. Further, Si is doped into the lower linear graded layer 15 to suppress the diffusion of Se into the n-type emitter layer 5 to have high reliability.
However, even when the Se is doped into the uniform composition layer 16 of the n-type InGaAs non-alloy layer 14 and the Si is doped into the linear graded layer 15 thereof, there is still room for improvement since the n-type InGaAs non-alloy layer 14 does not have a low contact resistance under some growth conditions on the In composition etc., and the flatness thereof is not adequate yet.
Further, when the Si is doped into the linear graded layer 15 of the n-type InGaAs non-alloy layer 14, it causes the excessive doping of the Se into the upper non-alloy layer (n-type InGaAs layer) 16 so as to reduce the contact resistance since the contact resistance of the linear graded layer 15 cannot be reduced completely. Thus, at the next epitaxial growth, the remaining Se may be mixed into the n-type sub-collector layer 2 and the n-type collector layer 3 as a memory material to cause an increase in, especially, collector capacitance. Furthermore, when the doping amount of Se is increased, a thorn-shaped projection 17 as shown in FIG. 8 can be generated in etching the non-alloy layer 16 by using an acid based etchant. Thus, reliability of the HBT will lower.